There's a new tweak called OkSiri that just came out in the BigBoss repo (and is free). You set it up using Activator. They recommend you use the AC power connect/disconnect. It works even when the device's screen is off. Pretty cool. I have my keyword setup as just "Siri", and I can be typing on my MacBook Pro, say SIRI, wait for it to pop up a second and then SET MY ALARM FOR 6AM!